Per the [Steam article on the subject][1]
> **Extra copies are not given in most cases.** 

> Only titles purchased in special promotions as outlined below are eligible for giftable extra copies

There has also been some discussion in gaming chat about the ethics of giving away extra humble bundle keys, as the site says the keys are for your sole use.

![Humble bundle description][2]

The original discussion was [here][3].  Ullallulloo emailed Humble Bundle support and asked for us.  He posted the reply in chat [here][4].  Here's what it said:

> "Our Steam keys do not generate giftable copies on Steam. Please
> consider your bundle a "unit" and not to share copies of the games or
> Steam keys from it.
> 
> If you'd like to give a copy of the games away, we have a system in
> place that allows you to buy a gift key from www.humblebundle.com so
> your friends can have access to direct downloads as well as any Steam
> or other keys we have included.
> 
> Please let me know if there's anything else I can help you with.
> 
> Thank you for your support"

For non-Humble Bundle trading, I would suggest the [meta post][5] related to game trading.


  [1]: https://support.steampowered.com/kb_article.php?ref=4502-TPJL-2656#how
  [2]: https://i.sstatic.net/EXdfh.png
  [3]: http://chat.stackexchange.com/transcript/message/2778038#2778038
  [4]: http://chat.stackexchange.com/transcript/message/2778712#2778712
  [5]: https://gaming.meta.stackexchange.com/questions/3536/steam-item-giveaway-trading-thread